Hull: Venice of the North? A dramatic sunset over the nearby Humber bridge. Photograph: John Giles/PA

Hull could be transformed into a Venice-like waterworld and Portsmouth into a south coast version of Amalfi, engineers and architects have claimed in a study of options for developing Britain's coastal cities in the face of rising sea levels.

The Institution of Civil Engineers and the Royal Institute of British Architects yesterday warned the future of cities including London, Bristol and Liverpool was at risk from seas which the Environment Agency predict could rise by as much as 1.9m by 2095 in the event of a dramatic melting of the Greenland ice sheet.

In a model that explores managed retreat from the coast in some areas, Hull's historic city centre would be limited to an island reached by bridges and Venetian-style water taxis, while in Portsmouth large parts of Portsea island would be given back to the sea while new "hillside living" developments would be built on densely packed hillside terraces, akin to the towns of Italy's Amalfi coast. "The scenarios we have created are extreme, but it is an extreme threat we are facing," said Ruth Reed, Riba president. "Approximately 10 million people live in flood-risk areas in England and Wales, with 2.6m properties directly at risk of flooding."

Other options include building out into rising waters using piers and platforms to create new habitable space – a strategy known as "attack". In Hull this could involve floating disused oil rigs up the Humber and reusing them for offices, homes and university buildings, while in Portsmouth two-storey piers could be built with the lower tier used for traffic and the top tier used for pedestrian space.

Architect David West, one of the report's author's, admitted the proposals were "blue sky thinking" and uncosted, but said they had the potential to relieve pressure for housing on inland sites. "I think the concept of arriving at Hull as if you were arriving at Venice airport and taking a boat into the city is really exciting."

The proposals were met with scepticism in Portsmouth. "A retreating coastline in this area would have a significant detrimental impact on the internationally designated harbours," said Bret Davies, a coastal strategy manager at Portsmouth city council.

The Environment Agency's coastal policy adviser, Nick Hardiman, warned that extending into the sea was likely to be too expensive and structures were not likely to be sustainable.In the next financial year the Environment Agency will spend £570m on building and maintaining flood defences.